Keyhole reports Lon 0 to a different place the visual markers at Greenwich. Check out the attachment and see the description below.

When you visit Greenwich there is a line on the ground (representation of longitude).

"The Meridian is marked in a number of places in Greenwich.

At the Observatory In the Meridian building and on its outside wall, in the courtyard (incorporating a sculpture) and on the path just outside. A green laser beam is projected from a box above Airy's transit circle after dark along the line of the meridian which can reach up to 15 miles.

Southwards There is a line of stones across the Avenue of the park. The meridian clips the raised mound covering a reservoir and bisects the putting green. There should be a stone in the rose beds by the Rangers House. The line goes through the dining room, crimson parlour and gallery of the house itself and is marked on the outside of the boundary wall (south of the house).

Northwards You can cross the meridian on the park's <b>Boating Lake!</b> The Millennium Sundial by the lake is some <b>2 metres OFF</b> the Meridian due to incorrect information being supplied. The diallist is unhappy about this as it makes the sundial incorrect by about 8 minutes but it is difficult to see how it could be rectified. In Park Row the meridian is marked on the wall of the Chantry and by a line of studs across the road. The Meridian School in Old Woolwich Road has a plaque on its wall. The line runs through the garden of Trinity Hospital (almshouses) opposite where it is marked on the path. It passes in front of the neighbouring power station, across the Thames and is recorded on the path around the Greenwich Peninsula. Two Meridian pubs are the Plume of Feathers just to the east in Park Row and the Yacht at 00° 00' 06" west in Crane Street.
Across the river from the Millennium Dome there is a metal strip at the Virginia Quay development."

http://www.london-footprints.co.uk/artmeridian.htm